Little is known about synaptic transmission reliability and uniformity in time and space. Moreover, the feedback system that matches innervation density with postsynaptic size is poorly understood. To address these questions, we have developed a method of measuring transmission at the level of single boutons in the larval Drosophila neuromuscular junction (NMJ), by using cameleon, a fluorescent calcium sensor. When expressed in the muscle, cameleon gives a fluorescent readout of synaptic activity with single bouton resolution. This system will be used to examine the efficacy of transmission in mutants involved in synaptic homeostasis. This is the process by which the presynaptic cells increase synaptic transmission to maintain adequate postsynaptic depolarization in the growing larval muscle. Because we have seen variability in synaptic efficacy in wildtype synapses, it is possible that the less efficient synapses, or even "silent synapses", are recruited and become more active as the muscle develops. Findings from these studies on Drosophila are relevant to the function and development of synapses in mammalian central nervous systems, since both systems use homologous molecules and show similar forms of plasticity.